Montes spied on her own country for Castro, doing much damage, yet Obama may soon liberate her.

The Obama administration is reportedly in secret negotiations with Cuba that would result in the release from federal prison of one of the most damaging American spies in U.S. history. Such an extraordinary gesture would be preposterous for many reasons.

Ana Belén Montes, who is serving a 25-year sentence as part of a 2002 plea deal, was a U.S. Justice Department official with a top-secret security clearance when she was approached by Cuban intelligence agents in 1984. At the time the Cuban regime ran a pervasive spying program against the U.S., as it still does today, though then it often acted in conjunction with the Soviet Union. A devoted sympathizer of radical Latin American regimes, Ms. Montes quickly agreed to spy for Havana, thus beginning a 16-year-long betrayal of the U.S.

As prosecutors later showed, Ms. Montes took a secret trip to Cuba to meet with her new spymasters, then sought government positions with greater access to classified information that would be useful to the Castro regime. In 1985 she began working for the Defense Intelligence Agency, which specializes in military intelligence. Ms. Montes quickly rose through DIA ranks, eventually becoming the agency’s leading Cuba analyst. She was granted access to top-secret classified information that she would memorize at work and type up at home, later passing the information to her Cuban handlers.

As I conveyed in a July 12 letter to President Obama, it is difficult to overstate the damage caused by Ms. Montes’s treachery. In May 2012, Michelle Van Cleave, the former head of U.S. counterintelligence who oversaw completion of the damage assessment on Ms. Montes, told Congress that her activities likely “contributed to the death and injury of American and pro-American forces in Latin America,” and that she compromised other, broader intelligence programs.

Nevertheless, press reports indicate that the Obama administration is considering releasing Ms. Montes to the Castro regime as part of a prisoner swap for American fugitives from justice now sheltered in Cuba.

This exchange would be part of the administration’s campaign to normalize ties with Cuba, which has included restoring diplomatic relations, loosening sanctions and removing Cuba from the list of state sponsors of terrorism. Hopes that the Castro regime would reciprocate by granting basic freedoms to the Cuban people and releasing political prisoners have gone unfulfilled.

The abundant incentives that President Obama offered to get Iran last year to sign a nuclear deal have already shown how far this administration will go to curry favor with hostile powers. As we saw in 2014 with the trade of five dangerous Taliban prisoners for Army Sgt. Bowe Bergdahl—now arraigned on charges of desertion and misbehavior before the enemy in Afghanistan—this president has odd ideas about what constitutes a beneficial prisoner swap. Even so, releasing Ms. Montes cannot be tolerated.

In the past, the U.S. has deported or traded captured foreign spies, but it is extremely rare to trade American citizens who have betrayed their country. Doing so would be especially egregious in these circumstances. The American government should not pay the Castro regime a bribe, in the form of a released American spy, in hopes of advancing normalization.

Ms. Montes’ release would send a dangerous message that convicted spies may be able to secure a deal through the foreign government that employed them. Potential traitors to this country should know that betraying America will bring harsh penalties, without exception or the potential for a get-out-of-jail-free card.

“Prison is one of the last places I would have ever chosen to be in, but some things in life are worth going to prison for,” an unrepentant Ms. Montes wrote to a relative, the Washington Post reported in 2013. If releasing American traitors from prison is the cost of “normalizing” relations with Cuba, then clearly that price is too high.